It is known that the catalytic converter effects an averaging on the oxygen content in the exhaust gas. If the exhaust gas flowing into the catalytic converter is rich in oxygen, then the catalytic converter can store the excess oxygen within certain limits and then output this excess oxygen when the exhaust gas input is deficient in oxygen. An oscillation of the oxygen content in the exhaust gas forward of the catalytic converter (as is typical for a known strategy for controlling the air/fuel ratio for an internal combustion engine) is therefore attenuated by the catalytic converter, that is, the oscillation is reduced with respect to its amplitude. If the exhaust-gas probe mounted rearward of the catalytic converter registers unwanted intense oscillations of the oxygen content, then this is evaluated as a sign for a defective catalytic converter because a loss of the capability of converting toxic substances is associated with the loss of this attenuating action.
In another known method, the actual value of the signal of an exhaust gas-probe, which is mounted downstream of the catalytic converter, is detected and an expectation value of the signal of the above-mentioned exhaust-gas probe is formed from quantities measured upstream of the catalytic converter. Furthermore, a measure for the deviation of the actual value from the expectation value is formed and the operability of the catalytic converter is judged on the basis of this deviation.
Uncertainties can occur in the evaluation of the operability of the catalytic converter in this method when using probes having a nonlinear characteristic and fluctuating control position, that is, a fluctuating mean value of the actual value of the oxygen construction in the exhaust gas.